1. Field of the Invention
This invention relates to a developing apparatus provided in an image forming apparatus such as a copier of the electro-photographic type or a printer.
2. Related Background Art
In the field of an image forming apparatus of the electrophotographic type, there is known and put into practical use an image forming apparatus in which an electrical latent image is formed on a photosensitive body which is a latent image bearing body utilizing a photoconductive substance by one of various methods, and then the latent image is developed by a developing apparatus with a toner which is a developer and is visualized as a toner image, and as required, the toner image is transferred to a transferring material which is a recording medium such as paper, whereafter heat and pressure or the like are imparted to the toner image to thereby fix the toner image and obtain a copy.
In such an image forming apparatus, however, improvements in the resolution, definition, etc. of image are now required and the development of a method of and an apparatus for forming a thin layer of toner on a developer carrying body provided in the developing apparatus is requisite, and several measures for this have been proposed.
In recent years, in such an image forming apparatus, there has been proposed a contact monocomponent developing method which adopts a semiconductive developing roller or a developing roller having a dielectric layer formed on the surface thereof as a developer carrying body carrying a developer thereon and in which this developing roller is urged against the surface layer of the photosensitive body with the developer interposed therebetween to thereby effect developing.
In this contact monocomponent developing method, developing is effected with the developing roller abutting against or brought into pressure contact with the photosensitive body bearing the electrostatic latent image thereon and therefore, it is necessary to use a developing roller having elasticity.
Particularly, when the developing roller is to be brought into contact with a latent image bearing body comprising a rigid body, it becomes a requisite condition to use a developing roller having elasticity in order to avoid injuring the latent image bearing body.
In such an image forming apparatus, however, the nonmagnetic monocomponent DC contact developing method according to the prior art suffers from the following problem.
When an electrically conductive developing roller is used, an electric current is created in the nip portion between the photosensitive body and the developing roller by the movement of the toner having charges, and developing is effected even at a low developing potential difference (the difference between the surface potential of the electrostatic latent image bearing body and the developing bias potential) and high image density is obtained.
Thus, there results image formation having a high .gamma. value, and the .gamma. value of developing obtained from the inclination of a curve resulting from output image density having been plotted relative to the developing potential difference becomes high, and it has been difficult to faithfully reproduce a halftone image conforming to the continuous image density of an original.